S. pombemeiotic linear elements contain proteins related to synaptonemal complex components

Abstract

IntroductionCells of sexually reproducing eukaryotes normally contain two equal (homologous) sets of chromosomes, one contributed by the father and the other by the mother during the fusion of gametes and the formation of a zygote. Meiosis is the cell division that reduces the number of chromosomes by half. It produces gametes or their precursor cells, each of which contains a haploid set consisting of randomly assorted parental chromosomes. These chromosomes are mosaics, because the original parental homologues have exchanged corresponding pieces by crossing-over. Thus, the function of meiosis is twofold -it compensates for the doubling of the chromosome number at fertilization and it provides the progeny with newly assorted sets of alleles, which is the basis of their genetic diversity.Crossing-over is initiated at multiple sites in recombining chromosomes by the enzymatic induction of double-strand breaks (DSBs). The resection of single strands at DSBs leads to the formation of gaps in the DNA. The missing bases are replenished by using the complementary sequence from the homologous chromosome as the template. This process is recombinogenic; that is, it can lead to the reciprocal exchange of DNA between the chromosomes involved (for review, see Keeney, 2001).In order to allow crossing-over, homologous parental chromosomes must pair during meiotic prophase via a ladderlike proteinaceous structure, the synaptonemal complex (SC) (for reviews, see Loidl, 1990;Zickler and Kleckner, 1999). The SC consists of two parallel axes (the lateral elements), to each of which the two chromatin threads of a single replicated chromosome are attached. The lateral elements are connected and kept at a distance of ~100 nm by the so-called transversal filaments. The ultrastructure of the SC is evolutionarily well conserved from protists to humans, although its molecular composition is far more heterogeneous.The fission yeast Schizosaccharomyces pombe features a meiosis that is unique in several respects. Most remarkably, it lacks an SC. Instead, so-called linear elements (LEs) appear during meiotic prophase (Olson et al., 1978;Bähler et al., 1993). LEs appear in the electron microscope (EM) as single lines of variable length, networks of interconnected lines or bundles of lines. These different morphological classes were found to prevail at different stages of meiotic prophase (Bähler et al., 1993), which suggests that their change in appearance is functionally related to chromosome pairing and/or recombination.Because a rec10 mutant lacks LEs, a structural or regulatory role of the Rec10 protein in LE formation has been proposed (Molnar et al., 2003). Apart from this indirect evidence, information on the molecular composition of LEs is scarce. Neither topoisomerase II nor Rec8 [which, in other organisms, constitute the cores along which lateral elements form (Klein et al., 1992;Klein et al., 1999)] delineate entire LEs (Hartsuiker et al., 1998;Parisi et al., 1999;Watanabe and Nurse, 1999), and S. pombe homologues of proteins prese...

Linear Element-Independent Meiotic Recombination in Schizosaccharomyces pombe

“…This is consistent with the observation that S. cerevisae Red1 null mutants are defective for recombination at some, but not all loci (Rockmill and Roeder 1990). The regional specificity of the rec10-109 mutant shows that Rec10 regulates recombination in the middle region of chromosomes, a pattern similar to that observed for the meiosis-specific cohesins, Rec8 and Rec11, although residual LinEs have been observed in this mutant, suggesting that it is not totally defective in LinE function (De Veaux and Smith 1994;Krawchuk et al 1999;Parisi et al 1999;Lorenz et al 2004). The similar regional pattern of recombination in the rec10-109 and cohesion mutants indicate that there is an intimate functional association between meiotic cohesins and LinEs.…”

“…The function of the SC remains poorly understood (Zickler and Kleckner 1999;Bishop and Zickler 2004). The fission yeast, Schizosaccharomyces pombe, is a member of a unique group of organisms that do not form fully mature SCs, although S. pombe does form cytologically distinct linear elements (LinEs) that exhibit similarities to AEs (Bähler et al 1993;Kohli and Bähler 1994;Lorenz et al 2004). The LinE structures of S. pombe and the AEs of Saccharomyces cerevisiae have conserved common features and the key structural components in the two organisms, Rec10 (S. pombe) and Red1 (S. cerevisiae), exhibit some amino acid conservation (Lorenz et al 2004).…”

“…The fission yeast, Schizosaccharomyces pombe, is a member of a unique group of organisms that do not form fully mature SCs, although S. pombe does form cytologically distinct linear elements (LinEs) that exhibit similarities to AEs (Bähler et al 1993;Kohli and Bähler 1994;Lorenz et al 2004). The LinE structures of S. pombe and the AEs of Saccharomyces cerevisiae have conserved common features and the key structural components in the two organisms, Rec10 (S. pombe) and Red1 (S. cerevisiae), exhibit some amino acid conservation (Lorenz et al 2004). This structural conservation and the absence of a visible SC in S. pombe strongly suggest that AEs serve some function other than providing a precursory platform for SC formation; they may function by contributing to the establishment of chromosomal mechanical forces of compaction, which have been proposed to regulate meiotic recombination (Blat et al 2002;Kleckner et al 2004).…”

“…Although LinE formation is rec10 1 dependent (Molnar et al 2003;Lorenz et al 2004), the precise function(s) of Rec10 and LinEs remains unclear. rec10 mutants have been isolated in a genetic screen for mutants that are defective in meiotic recombination, indicating that Rec10 is required for this process (Ponticelli and Smith 1989;De Veaux et al 1992).…”

“…Microscopical preparation and staining: Meiotic time courses and nuclear spreads were carried out as previously described (Lorenz et al 2004). In short, cells from sporulating cultures were freed of cell walls by enzymatic treatment, applied to a microscopic slide, opened up with a detergent, and fixed with paraformaldehyde.…”